This invention relates generally to flame detector provided to boilers and the like, and particularly to such flame detector which detects burner flame optically.
In various fuel combustion apparatus, such as large-sized boilers for factories or the like, it is important to detect accurately the state of combustion so as to ensure the safety and economy.
In the case of a boiler used for a power plant, it is desired that the reliability in detection of burner flame is further increased because of various circumferences, such as the increase in the number of burners caused from the tendency of increasing the capacity of boilers, the increase in the number of times of igniting and fire extinguishing caused from DSS (daily start-stop) operation, the adoption of the way of surpressing the generation of oxygen nitride (NOx) and the variation in the combustion state due to the diversification of used fuel.
Methods for detecting flame may be devided into two, one being ion method and the other being an optical method. In ion method, since flame electrode is used as a sensor, the flame detector cannot be used continuously for a long period of time, and therefore, such type flame detector is limited generally to flame detectors for use with igniting burners. On the other hand, optical flame detectors are arranged to detect combustion state and extinction state by detecting the intensity of radiated light from the flame, and are divided into two types, one being a system of detecting used wavelength and direct current light and the other being a system of determined flicker.
FIG. 31 shows a general structure of a conventional flame detector of optical type. As shown, one or more heat-resisting mirrors 103 are disposed within a hollow cylindrical light guiding tube 102 having an opening 101 at its side wall. Light ray 105 from unshown flame is led via the opening 101 into the light guiding tube 102, and is reflected at the mirror 102 to be incident on a sensor 104 such as a silicone photocell. Then the light is converted into an electrical signal to be outputted from the sensor 104 to be applied to an unshown control unit.
While this conventional flame detector is simple in construction, there is a drawback that the field of vision for flame detection is limited to a relatively narrow range. Since primary combustion zone where the brightness of the flame is high, moves back and forth relative to the tip of a burner due to the variation in load, such narrow field of vision for flame detection results in low accuracy in detection. Furthermore, such narrow field of vision makes it difficult to distinguish one flame of an objective burner from light of other flames. More specifically, in conventional arrangement of this type, the intensity of detected light is simply compared with a predetermined threshold provided for flame detection, and when the detected intensity is below the threshold, it is detrmined that there exists no flame, i.e the state of extinction, and on the other hand, when the detected intensity is above the threshold, then it is determined that there exists a flame, i.e. state of combustion, and such detection signal is fed to an alarm, a monitor and so on.
In more complex conventional flame detectors, the value of the threshold for the flame detection is changed in accordance with the sort of fuel to be combusted. However, only one threshold is used for each sort of fuel. Therefore, detection accuracy does not increase although various sorts of fuel can be used. Furthermore, detecting operation would rather be complex with such variable threshold.
Moreover, in actual flame detection, determination logic, in which higher intensity of flame light means combusting state, is not always correct because there occurs a phenomenon that higher intensity of flame light is detected in connection with a burner which is in extinction state.
This phenomenon will be described with reference to FIGS. 32 and 33. In the case of detecting whether an ignition burner 106 is in combustion state or extinction state by way of a flame detector 107, when the ignition burner 106 is in extinction state as shown in FIG. 32, a large amount of light from a flame F1 of a main burner 108 is detected by the flame detector 107, and the amount of light received by the sensor of the flame detector 107 becomes large. On the other hand, when the ignition burner 106 is in combustion state, the light of the flame F2 of the main burner 108 is shut off by the flame F1 of the ignition burner 106, and therefore, the amount of received light is reduced. For this reason, the above-mentioned determination logic is apt to result in erroneous determination with which a false signal is fed to fuel supply system or the like.
Such erroneous detection may occur not only between an ignition burner and a main burner but also between a plurality of burners disposed nearby such as in a cell burner.